Robust catheter tubing

ABSTRACT

A balloon catheter having a multilayer catheter shaft is formed to have an inner layer and an outer layer, where the inner layer and outer layer are selected from materials that enhance the pushability of the catheter while preserving the flexibility. Using a combination of a high Shore D duromater value material and a lower Shore D duromater value material, various combinations of multilayer catheter shafts are disclosed utilizing different glass transition temperatures and block copolyamides to obtain the desired characteristics.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/478,929, filed Jun. 5, 2009, which is continuation-in-part of U.S.patent application Ser. No. 12/324,425 entitled “Low Compliant CatheterTubing” filed Nov. 26, 2008, now U.S. Pat. No. 8,070,719, the contentseach of which are incorporated by reference in their entirety.

BACKGROUND

The invention relates to the field of intravascular catheters, and moreparticularly to a balloon catheter or other catheter components, such asa guidewire enclosure and catheter tubing, that would benefit from theproperties of the materials disclosed herein.

In percutaneous transluminal coronary angioplasty (PTCA) procedures, aguiding catheter is advanced until the distal tip of the guidingcatheter is seated in the ostium of a desired coronary artery. Aguidewire, positioned within an inner lumen of an dilatation catheter,is first advanced out of the distal end of the guiding catheter into thepatient's coronary artery until the distal end of the guidewire crossesa lesion to be dilated. Then the dilatation catheter having aninflatable balloon on the distal portion thereof is advanced into thepatient's coronary anatomy, over the previously introduced guidewire,until the balloon of the dilatation catheter is properly positionedacross the lesion. Once properly positioned, the dilatation balloon isinflated with liquid one or more times to a predetermined size atrelatively high pressures (e.g. greater than 8 atmospheres) so that thestenosis is compressed against the arterial wall and the wall expandedto open up the passageway. Generally, the inflated diameter of theballoon is approximately the same diameter as the native diameter of thebody lumen being dilated so as to complete the dilatation but notoverexpand the artery wall. The rate of expansion of the balloon for agiven pressure is an important consideration in the design of thedilation catheter, as greater than anticipated expansion of the balloonagainst the vessel wall can cause trauma to the vessel wall. After theballoon is finally deflated, blood flow resumes through the dilatedartery and the dilatation catheter can be removed from the patient'sartery.

In such angioplasty procedures, there may be restenosis of the artery,i.e. reformation of the arterial blockage, which necessitates eitheranother angioplasty procedure, or some other method of repairing orstrengthening the dilated area. To reduce the restenosis rate and tostrengthen the dilated area, physicians frequently implant anintravascular prosthesis, generally called a stent, inside the artery atthe site of the lesion. Stents may also be used to repair vessels havingan intimal flap or dissection or to generally strengthen a weakenedsection of a vessel. Stents are usually delivered to a desired locationwithin a coronary artery in a contracted condition on a balloon of acatheter which is similar in many respects to a balloon angioplastycatheter, and expanded to a larger diameter by expansion of the balloon.The balloon is deflated to remove the catheter and the stent left inplace within the artery at the site of the dilated lesion.

In the design of catheter balloons and catheter tubing, materialcharacteristics such as strength, flexibility and compliance must betailored to provide optimal performance for a particular application.Angioplasty balloons and catheter tubing preferably have high strengthfor inflation at relatively high pressure, and high flexibility andsoftness for improved ability to track the tortuous anatomy. The ballooncompliance, for example, is chosen so that the balloon will have adesired amount of expansion during inflation. Compliant balloons, forexample balloons made from materials such as polyethylene, exhibitsubstantial stretching upon the application of tensile force.Noncompliant balloons, for example balloons made from materials such asPET, exhibit relatively little stretching during inflation, andtherefore provide controlled radial growth in response to an increase ininflation pressure within the working pressure range. However,noncompliant balloons generally have relatively low flexibility andsoftness, making it more difficult to maneuver through various bodylumens. Heretofore the art has lacked an optimum combination ofstrength, flexibility, and compliance, and particularly a low tonon-compliant balloon with high flexibility and softness for enhancedtrackability. Semi-compliant balloons made from semi-crystalline nylon11, nylon 12, and copolymers of these nylons, such as poly ether blockamide (for example, Pebax from Arkema) address these shortcomings andprovide low distensibility and good flexibility, thus being used in manyballoon dilatation catheters and stent delivery system.

For ease of thermal bonding to afore mentioned semi-compliant balloons,it is also preferred that the shaft of the balloon dilatation cathetersand stent delivery system are also derived from same materials. Manyballoon dilatation catheters and stent delivery systems therefore useshafts derived from these materials. The relative hardness andflexibility of the catheter tubing is also a constant compromise betweenthe need for an agile tubing that can navigate the various body lumens,while having enough stiffness to be able to be pushed from a proximalend outside the body through the patient's vascular tract. A tubing thatis relatively stiff will transmit proximal force more efficiently to thedistal end, giving the practitioner more control over the location andposition of the catheter balloon. However, stiffer tubing makes it muchmore difficult to bend or track curvatures in the body, leading to theparadox of the need for stiffer yet more flexible tubing. Moreover,stiffer materials when used to make catheter shafts have a highertendency to kink, making it more difficult to control or push.Therefore, it is desirable to be able to increase stiffness and thuspushability by incorporating nylons having higher stiffness thansemi-crystalline nylons having been used thus far. Some amorphous nylonsoffer desired higher stiffness than semi-crystalline nylons. However,the higher stiffness amorphous nylons are more susceptible to damagefrom solvents, such as isopropyl alcohol, used in the manufacturing,cleaning processes, and during clinical use. The present invention isdirected to address this issue.

SUMMARY OF THE INVENTION

The softness and flexibility of a balloon or catheter tubing is afunction of the flexural modulus of the polymeric material of theballoon, so that a balloon or tubing material having a higher Shore Ddurometer hardness, which yields a stronger and stiffer balloon orcatheter tubing, has a higher flexural modulus. Conversely, a balloon orcatheter tubing material having a lower Shore D durometer hardness,which thus provides a soft and flexible balloon or tubing, has a lowerflexural modulus. The present invention is directed to a catheter tubingformed with a combination of at least two polyamides, a high durometerhardness material and a lower durometer hardness material.

The tubing can be made from a blend of the two polyamides, or aco-extrusion of the two polyamides with an inner layer and an outerlayer. The first inner polyamide has a Shore D durometer hardness ofgreater than 78 D, more preferably Shore D durometer hardness of greaterthan 80 D, and can be preferably selected from various transparentamorphous nylons having segment such as an aliphatic segment, anaromatic segment, or a cycloaliphatic segment. The second outerpolyamide has a lower durometer hardness than the first polyamide, andpreferably less than 76 D, and preferably a block copolymer of nylon andpolytetramethylene oxide (i.e. a copolyamide), or Pebax. The secondouter polyamide is preferably semi-crystalline polyamide, thus providingenhanced resistance to solvents, such as isopropyl alcohol, used in themanufacturing, cleaning processes, and during clinical use. Both innerand outer polyamides preferably have the same amide block or segment,e.g. nylon 12, nylon 11, or nylon 6,6.

The preferred high hardness material is a nylon referred to astransparent amorphous nylon. The transparent amorphous nylon preferablyhas either an aliphatic segment, an aromatic segment, or acycloaliphatic segment.

The catheter tubing of the invention may be formed by coextruding atubular product formed from the two polymeric components to create atubing having an outer layer and an inner layer of the two materials.

Various designs for balloon catheters well known in the art may be usedin the catheter system of the invention. For example, conventionalover-the-wire balloon catheters for angioplasty or stent deliveryusually include a guidewire receiving lumen extending the length of thecatheter shaft from a guidewire port in the proximal end of the shaft.Rapid exchange balloon catheters for similar procedures generallyinclude a short guidewire lumen extending to the distal end of the shaftfrom a guidewire port located distal to the proximal end of the shaft.

In the case of a robust co-extruded catheter shaft, an inner layer canbe comprised of a high modulus or high Shore D durometer nylon,preferably a Shore D durometer value greater than 80, such as GrilamidTR55 LX nylon 12 from EMS—American Grilon Inc., which provides a highmodulus for enhanced pushability. Also, it would be beneficial if theglass transition temperature of the high modulus polymer forming theinner layer is greater than 100° C., and more preferably greater than120° C. However, the TR55 LX or other high modulus amorphous nylon has agreater tendency to kink and is susceptible to attack by solvents. Tocombat the kink propensity and to protect the catheter shaft fromsolvents, a thin outer layer of lower modulus material such as Pebax isformed over the TR55 LX inner layer to serve as a protective layer andto resist kinking of the shaft. The outer layer can be selected fromsemi-crystalline polyamide or more preferably from a copolyamide oflower durometer value such as Pebax 72 D, Pebax 63 D, and the like,where the Shore D durometer value is less than 76.

By incorporating an amorphous polyamide such as TR 55 LX as one layer,the dimensional stability is increased for as the shelf life of thecatheter increases due to the higher glass transition temperature, ascompared with nylon 12 or Pebax which has a glass transition temperatureat or below 55° C. Also, a higher tensile strength (approximately 11,000psi) of amorphous nylon TR 55 LX compared with nylon 12 or Pebax(7,500-8,200) provides a higher rupture limit with the same wallthickness.

The co-axial tubing can be formed into a tapered configuration, wherethe radius of the shaft gradually reduces until the balloon attachmentpoint. This gradual reduction in the radial component of the shaftserves to eliminate the mid-shaft portion of the catheter, whichsimplifies the manufacturing process by avoiding a laser operation orother mechanism to attach the mid-shaft.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevated view partially in section of a balloon catheterwhich embodies features of the invention, showing the balloon in anexpanded state;

FIG. 2 is a transverse cross sectional view of the balloon catheter ofFIG. 1 taken along lines 2-2;

FIG. 3 is a transverse cross sectional view of the balloon catheter ofFIG. 1 taken along lines 3-3; and

FIG. 4 is a graph of the compliance of the catheter balloon using afirst preferred blend of materials;

FIG. 5 is a cross-sectional view of a catheter shaft with an innerlayer, an intermediate layer, and an outer layer;

FIG. 6 is an enlarged, partially cut-away view of a catheter shaftillustrating multiple layers, including an embedded high strength ribbonor coil in the outer layer; and

FIG. 7 is an enlarged, cross-sectional view of another embodiment of acatheter shaft.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In angioplasty balloons, an object is to apply a high pressure to theinterior wall of the lumen to compress the plaque and/or to fully expandthe stent. This relies on a robust balloon with a sturdy out wall and ahigh pressure capability. The compliance of the balloon, i.e., theexpansion of the balloon as a function of internal pressure, ispreferably low or flat to more accurately control the amount of pressureapplied to the arterial wall. However, the deliverability of the balloonis also a factor, especially where tortuous body lumens are involved.Stiff balloons (i.e., high modulus materials) tend to have poorflexibility and lack the maneuverability to navigate the various bodylumens, and thus make poor choices for catheter balloons. Conversely,flexible balloons (low modulus materials) that have high compliance arepoorly suited to apply a precise known pressure on the arterial wall dueto a high expansion rate per applied pressure. The goal is thus toincrease the rupture strength by adding the high modulus material suchas nylon to the softer polyamide material.

Soft polyamide materials such as Pebax® are semi-crystalline polymersand usually include an amorphous segment. The amorphous segment has alower density than the crystalline structure and thus is weaker ingeneral than crystalline segments. If the amorphous segment can bereinforced by adding a small amount of a higher modulus material theresponse of the amorphous segment can be delayed and the overallstrength of the material can be strengthened. The high modulus materialpreferably has a Shore D durometer hardness of 78 D or more. Suitablematerials include transparent amorphous nylon such as nylon 12, and morepreferably a nylon 12 with a aliphatic segment, an aromatic segment, ora cycloaliphatic segment. These nylons are transparent amorphous becausethey are essentially amorphous, lacking the crystalline structure ofother more conventional nylon 12. The aliphatic segment, aromaticsegment, or cycloaliphatic segment does not crystallize with the mainchain, disrupting the formation of longer crystalline chains in thepolymer. The amorphous segment of the transparent amorphous nylon 12combines with the amorphous segment of the Pebax to strengthen the Pebaxby enhancing the weakest link in the chain, thereby increasing theoverall strength of the polymer.

FIG. 1 illustrates a balloon catheter which embodies features of theinvention. The catheter 10 of the invention generally comprises anelongated catheter shaft 11 having a proximal section, 12 a distalsection 13, an inflatable balloon 14 formed of a blend of polymericmaterials on the distal section 13 of the catheter shaft 11, and anadapter 17 mounted on the proximal section 12 of shaft 11. In FIG. 1,the catheter 10 is illustrated within a patient's body lumen 18, priorto expansion of the balloon 14.

In the embodiment illustrated in FIG. 1, the catheter shaft 11 has anouter tubular member 19 and an inner tubular member 20 disposed withinthe outer tubular member and defining, with the outer tubular member,inflation lumen 21. Inflation lumen 21 is in fluid communication withthe interior chamber 15 of the inflatable balloon 14. The inner tubularmember 20 has an inner lumen 22 extending therein which is configured toslidably receive a guidewire 23 suitable for advancement through apatient's coronary arteries. The distal extremity of the inflatableballoon 14 is sealingly secured to the distal extremity of the innertubular member 20 and the proximal extremity of the balloon is sealinglysecured to the distal extremity of the outer tubular member 19.

FIGS. 2 and 3 show transverse cross sections of the catheter shaft 11and balloon 14, respectively, illustrating the guidewire receiving lumen22 of the guidewire's inner tubular member 20 and inflation lumen 21leading to the balloon interior 15. The balloon 14 can be inflated byradiopaque fluid introduced at the port in the side arm 24 intoinflation lumen 21 contained in the catheter shaft 11, or by othermeans, such as from a passageway formed between the outside of thecatheter shaft and the member forming the balloon, depending on theparticular design of the catheter. The details and mechanics of ballooninflation vary according to the specific design of the catheter, and arewell known in the art.

Non-compliant or low-compliant balloon 14 and/or the shaft 11 is formedof a blend of a first polyamide having a Shore D durometer hardnessgreater than 78 D and a copolyamide of lower durometer hardness,preferably less than 76 D. A preferred polyamide having a Shore Ddurometer hardness greater than 78 D is an amorphous polyamide such asEMS TR 55 (transparent amorphous nylon 12), Arkema Rilsan G110(transparent amorphous nylon 12), or Cristamid MS 110 (transparentamorphous nylon 12). The polyamide is preferably includes acycloaliphatic segment, an aromatic segment, or an aliphatic segment.Such polyamides are also referred to as transparent polyamide. Thepreferred copolyamide material for forming the polymeric blend for theballoon is Pebax, and more preferably Pebax 72 D, Pebax 70 D, Pebax 63D, or Pebax 55 D. Alternatively, the copolyamide of lower durometerhardness is preferably a block copolymer of nylon 12 andpolytetramethylene oxide.

The flexural modulus of the polyamide is preferably greater than 1700MPa (240,000 psi) and the flexural modulus of the copolyamide is lessthan 850 MPa (120,000 psi). The tensile strength at break of bothpolyamides is at least 50 MPa, and elongation at break of bothpolyamides is at least 150%.

The catheter shaft will generally have the dimensions of conventionaldilatation or stent deploying catheters. The length of the catheter 10may be about 90 cm to about 150 cm, and is typically about 135 cm. Theouter tubular member 19 has a length of about 25 cm to about 40 cm, anouter diameter (OD) of about 0.039 in to about 0.042 in, and an innerdiameter (ID) of about 0.032 in. The inner tubular member 20 has alength of about 25 cm to about 40 cm, an OD of about 0.024 in and an IDof about 0.018 in. The inner and outer tubular members may taper in thedistal section to a smaller OD or ID.

The length of the compliant balloon 14 may be about 1 cm to about 4 cm,preferably about 0.8 cm to about 4.0 cm, and is typically about 2.0 cm.In an expanded state, at nominal pressure of about 8 to about 10 atm,the balloon diameter is generally about 0.06 in (1.5 mm) to about 0.20in (5.0 mm). and the wall thickness is about 0.0006 in (0.015 mm) toabout 0.001 in (0.025 mm), or a dual wall thickness of about 0.025 mm toabout 0.056 mm. The burst pressure is typically about 20 to 26 atm, andthe rated burst pressure is typically about 18 atm.

In a presently preferred embodiment, the balloon 14 may include wings,which may be folded into a low profile configuration (not shown) forintroduction into and advancement within the patient's vasculature. Wheninflating the balloon to dilate a stenosis, the catheter 10 is insertedinto a patient's vasculature to the desired location, and inflationfluid is delivered through the inflation lumen 21 to the balloon 14through the inflation port 24. The semi-compliant or noncompliantballoon 14 expands in a controlled fashion with limited radialexpansion, to increase the size of the passageway through the stenosedregion. Similarly, the balloon has low axial growth during inflation, toa rated burst pressure of about 14 atm, of about 5 to about 10%. Theballoon is then deflated to allow the catheter to be withdrawn. Theballoon may be used to deliver a stent (not shown), which may be any ofa variety of stent materials and forms designed to be implanted by anexpanding member, see for example U.S. Pat. No. 5,514,154 (Lau et al.)and U.S. Pat. No. 5,443,500 (Sigwart), incorporated herein in theirentireties by reference.

EXAMPLE 1

A proximal shaft for the over-the-wire catheter may have a taperedtubing coextruded and tapered with TR55 inner layer and Pebax 72 D outerlayer. The proximal wall thickness of TR55 may be approximately 0.005″and a proximal wall thickness of Pebax 72 D may be approximately 0.001″.A distal wall thickness of TR55 is approximately 0.002″ and a distalwall thickness of Pebax 72 D is approximately 0.001″. In addition toballoons, the blended composition has usefulness as other parts of thecatheter, such as the guidewire enclosure 20 of FIGS. 1-3. The innermember of the multi-layered tubing can have a lubricious inner layer(HDPE. UHMWPE, and the like) with bonding mid layer and polymer blendouter layer. Like the catheter balloon, the blend is comprised of onepolymer having a Shore D durometer greater than 78 and another polymerhaving lower durometer, preferably less than 76 D. Both polyamidespreferably have same amide block or segment, i.e. one type of amide(nylon) block, solely comprised of nylon 12, nylon 11, nylon 6, or nylon6, 6 but not combination of these.

The polyamide having Shore D durometer greater than 78 D is preferablyamorphous polyamide selected from polyamide such as EMS TR 55(transparent amorphous nylon 12), Arkema Rilsan G110 (transparentamorphous nylon 12), or Cristamid MS 110 (transparent amorphous nylon12). This polyamide is preferably a copolyamide comprisingcycloaliphatic, and/or aromatic, and/or aliphatic segment. The othercopolyamide of lower durometer is preferably a block copolymer of nylon12 and polytetramethylene oxide, such as Pebax 72 D, Pebax 70 D or Pebax63 D.

The high durometer polymer serves to increase resistance to collapse ofthe tubing and provides enhanced pushability while the lower durometerpolymer provides flexibility and kink resistance. Although it ispreferred to have blends of high miscibility, the blend ratio is suchthat the lower durometer polymer forms a “virtual” continuous phasewhile the higher durometer polymer forms “virtual” reinforcement.

As shown in FIG. 5, an outer layer 60 of the catheter shaft can becomprised of an amorphous polyamide selected from polyamide such as EMSTR 55 (transparent amorphous nylon 12), Arkema Rilsan G110 (transparentamorphous nylon 12), Cristamid MS 110 (transparent amorphous nylon 12),polyamide 11, polyamide 6, or polyamide 6,6. This polyamide ispreferably a copolyamide comprising cycloaliphatic, and/or aromatic,and/or aliphatic segment. In one embodiment, the outer layer 60 isblended with a softer polyamide such as a crystalline orsemi-crystalline copolymer of nylon 12 and polytetramethylene oxide orpolytetramethylene glycol, e.g. Pebax 72 D or Pebax 70 D. This blendingoffers a higher strength outer layer that offers higher pushability andresists collapse, while the copolymer operates to resist kinking andyield greater flexibility. Although it is preferable to have blends ofhigh miscibility, blend ratios are such that the lower durometer polymerforms a virtual continuous phase and the high durometer polymer forms avirtual reinforcement. The intermediate layer 70 is a bonding layer,such as Primacore, to meld the inner and outer layers together. Theinner layer 80 can be a lubricious material that reduces the friction ofa guide wire passing through the lumen 90, such as HDPE or UHMWPE. Othermaterials are also contemplated, as long as the outer layer has a glasstransition or melting temperature that is preferably lower than, or atleast approximately equal to, the surface temperature of the mold duringthe blowing or forming process of the balloon.

FIG. 6 shows another embodiment of a catheter shaft 120 having a lumen125 extending therein, the shaft 120 having an inner layer 130comprising a low friction material such as high density polyethylene andan intermediate layer 140 serving as a bonding layer. An outer layercomprising a first polymer having a first Shore D durometer value of nogreater than 76 D, such as Pebax 72 D or Pebax 63 D is impregnated witha second polymer having a Shore D durometer value that is greater thanthe first Shore D durometer value, and preferably greater than 78 suchas TR55 or the other transparent nylon 12 materials discussed above. Thesecond polymer 160 can be wrapped around the shaft 120 like a helix orcoil to reinforce the outer layer 150.

FIG. 7 illustrates another embodiment of a catheter shaft having aninner layer 220 of a high modulus material with a high Shore D durometervalue such as amorphous nylon 12. This inner layer material provides ahigh strength catheter shaft for higher pushability. However, thetransparent nylon 12 has a tendency to kink, and it is susceptible tosolvents used in cleaning or other manufacturing processes. To resistkinking, and to protect the catheter from solvents, a thin outer layer240 of Pebax or other soft copolyamide is formed over the inner layer220. The two layers 220, 240 can be co-extruded in a single operation tocreate the shaft 200 of FIG. 7. The shaft 220 also incorporates atapered section 250 as the shaft transitions from a main body portion tothe portion 260 to the portion 270 where a balloon may be attached. Inexisting catheter shafts, this section requires a mid shaft section thatmust be separately attached to the catheter shaft using expensive laserequipment. The tapered transition from the main body portion 260 to theballoon attachment portion 270 eliminates the need for a mid shaft andthus the need for the laser equipment and assembly line personnelrequired to operate the laser equipment, resulting in a more efficientand cost effective catheter.

The shaft of FIG. 7 preferably has both co-extruded layers 220, 240 witha common amide block or segment for better adhesion or compatibility,i.e., one type of amide (nylon) block, comprising one and only one ofnylon 12, nylon 11, nylon 6, or nylon 6,6. The inner polymer ispreferably a polyamide having a Shore D durometer value of greater than78, such as amorphous polyamide selected from EMS TR55, Arkema RilsanG110, or Cristamid MS 110, all commonly referred to as transparent nylon12. This polyamide preferably comprises cycloaliphatic, aromatic, and/oraliphatic segments. The outer layer 240 may preferably be a copolyamideblock copolymer of nylon 12 and polytetramethylene glycol.

Various embodiments are described above in effort to illustrate theconcepts of the present invention, but these embodiments are notintended to be limiting or exclusive. Rather, the scope of the inventionis to be determined by the words of the appended claims, interpreted inthe context of the above description but not limited to those examplesand embodiments described above and shown in the Figures.

We claim:
 1. A method of forming an elongated, flexible catheter,comprising: forming a shaft having a proximal end, a distal end, and alumen extending therein, the shaft having an inner layer comprising anamorphous polyamide having a Shore D durometer value of greater than 78,and an outer layer comprising a semi-crystalline polyamide orcopolyamide having a Shore D durometer value of less than 76, whereinthe outer layer is thinner than the inner layer, the shaft having anincreased resistance to kinking as compared to a similar shaft of asingle layer of the amorphous polyamide having a Shore D durometer valueof greater than 78; and providing a working device disposed on thedistal end of the shaft.
 2. The method of claim 1, wherein the workingdevice is an inflatable balloon.
 3. The method of claim 1, wherein theamorphous polyamide of the inner layer has a glass transitiontemperature of greater than 100° C.
 4. The method of claim 1, whereinthe polyamide or copolyamide of the outer layer has a glass transitiontemperature of less than 55° C.
 5. The method of claim 1, wherein theamorphous polyamide of the inner layer and the polyamide or copolyamideof the outer layer have a common amide block.
 6. The method of claim 5,wherein the common amide block is selected from the group consisting ofnylon 12, nylon 11, nylon 6, and nylon 6,6.
 7. The method of claim 1,wherein the inner polyamide includes at least one segment selected fromthe group consisting of cycloaliphatic segments, aliphatic segments, andaromatic segments.
 8. The method of claim 1, wherein the inner polyamidecomprises at least two segments selected from the group consisting ofaliphatic segments, aromatic segments, and cycloaliphatic segments. 9.The method of claim 1, wherein the inner and outer layers are fowled byco-extrusion.
 10. The method of claim 9, wherein the catheter shaft istapered.
 11. The method of claim 9, wherein the catheter shaft has anecked diameter portion.
 12. The method of claim 1, wherein the outerlayer is a polymer of nylon or a block copolymer of nylon.
 13. Themethod of claim 12, wherein the nylon is nylon 11, nylon 12, nylon 6,nylon 6,6 or nylon 6,12.
 14. The method of claim 12, wherein the blockcopolymer is a block copolymer of nylon and polytetramethylene oxide.15. The method of claim 14, wherein the nylon is nylon 11, nylon 12,nylon 6, nylon 6,6 or nylon 6,12.
 16. The method of claim 1, wherein theouter layer comprises a block copolymer of nylon 12 andpolytetramethylene oxide.
 17. The method of claim 1, wherein the shafthas an enhanced resistance to solvents as compared to a similar shaft ofa single layer of the amorphous polyamide having a Shore D durometervalue of greater than
 78. 18. The method of claim 1, wherein the outerlayer is thinner than the inner layer along substantially an entirelength of the shaft extending from the proximal end to the distal end.19. A method of forming an elongated, flexible catheter, comprising:forming a shaft having a proximal end, a distal end, and a lumenextending therein, the shaft having an inner layer comprising highdensity polyethylene and an outer comprising a blend of two polymers,including a first polymer being an amorphous nylon having a Shore Ddurometer value of greater than 78 and a second polymer having a Shore Ddurometer value of no more than 76, wherein the inner layer is thickerthan the outer layer, the shaft having an increased resistance tokinking as compared to a similar shaft having an inner layer of the highdensity polyethylene and an outer layer of the amorphous nylon having aShore D durometer value of greater than 78; and providing a workingdevice disposed on the distal end of the shaft.
 20. The method of claim19, further comprising forming an intermediate layer disposed betweenthe inner layer and the outer layer, the intermediate layer serving as abonding layer.
 21. The method of claim 19, wherein the first polymer andthe second polymer have a common amide block.
 22. The method of claim21, wherein the common amide block is selected from the group consistingof nylon 12, nylon 11, nylon 6, and nylon 6,6.
 23. The method of claim19, wherein the first polymer includes at least one segment selectedfrom the group consisting of cycloaliphatic segments, aliphaticsegments, and aromatic segments.
 24. The method of claim 19, wherein thefirst polymer comprises at least two segments selected from the groupconsisting of aliphatic segments, aromatic segments, and cycloaliphaticsegments.
 25. The method of claim 19, wherein the inner layer and outerlayer are formed by co-extrusion.
 26. The method of claim 19, whereinthe second polymer is a semi-crystalline polymer of nylon or a blockcopolymer of nylon.
 27. The method of claim 26 wherein the nylon isnylon 11, nylon 12, nylon 6, nylon 6,6 or nylon 6,12.
 28. The method ofclaim 19, wherein the block copolymer is a block copolymer of nylon andpolytetramethylene oxide.
 29. The method of claim 19, wherein the shafthas an enhanced resistance to solvents as compared to a similar shafthaving an inner layer of the high density polyethylene and an outerlayer of the amorphous nylon having a Shore D durometer value of greaterthan
 78. 30. The method of claim 19, wherein the outer layer is thinnerthan the inner layer along substantially an entire length of the shaftextending from the proximal end to the distal end.